Magnetic media, such as computer hard disks, rely on magnetic properties to store data. To ensure that the disks meet certain standards of quality, these magnetic properties need to be tested from time to time, particularly during the manufacturing process.
Two important magnetic properties of interest include remanence and coercivity. Coercivity (typically expressed in oersteds) refers to the intensity of an applied magnetic field required to reduce the magnetization of a sample of ferromagnetic material to zero after the magnetization of the sample has been driven to saturation. For its part, remanence (typically expressed in ampere/m) refers to the magnetization left behind in the sample after an external magnetic field is removed.
The coercivity and remanence of a disk can be measured in the following way. A magnetic field is applied to the disk with a certain sweep rate from a positive value to a negative value and then back to its original positive value. A polarized laser beam is shone towards the disk surface at a point of interest that is subjected to the magnetic field. The beam is reflected by the disk surface. Due to what is known as the magneto-optic Kerr effect (MOKE), the reflected beam will undergo an amount of polarization rotation that depends on the magnetic properties of the disk surface. The component of the reflected beam exhibiting a changed polarization angle is known as the “Kerr signal”. The Kerr signal is sampled many times during the sweeping of the magnetic field and a hysteresis loop is plotted from the acquired measurements. From this, the corecivity and remanence of the magnetic disk (at the point of interest) can be determined. To determine the coercivity and remanence of the magnetic disk at a second or subsequent point of interest on the surface of the disk, the disk position is changed and the process is repeated.
Since sweeping of the magnetic field takes several seconds, it is clear that testing numerous points of interest (e.g., different sectors of a track) can take minutes if not hours. This length of time for testing a single disk can be considered inefficient. Thus, there is a need in the industry for an apparatus and method for testing the magnetic properties of a magnetic medium by virtue of which multiple regions of the medium can be tested more efficiently.